The role of low threshold metastable excitations in the kinetics of a molecular oxygen plasma generated by a particle beam

A general formalism has already been derived for the inversion of the Boltzmann high-energy operator in particle-beam-sustained molecular plasma discharge. This inversion is possible provided the separation between 'bulk' (elastic processes dominant) and 'tail' (inelastic processes dominant) of the electron distribution function occurs very suddenly. Most molecular plasmas satisfy this model. However, referring to the scaling laws derived from this formalism, molecular oxygen appears as an important exception. Owing to low threshold metastable state excitations, the 'bulk' and the 'tail' of the electron distribution function are separated by an intermediate region where elastic and inelastic collisions are equally important. This problem is solved analytically.